The sub-field encoding part of a display using PWM technology is one of the most important parts of the display device since the encoding is responsible of the gray-scale portrayal (linearity and level of noise dithering) and of the motion rendition (level of false contour).
The goal of the sub-field encoding is to fill up a sub-fields memory with subfields data. The subfield data of a pixel is a code word wherein each bit is representative of the state, “ON” or “OFF”, of this pixel during a subfield of the video frame. This sub-fields memory will be read during the next frame, sub-field by sub-field, whereas it is written pixel by pixel. This information is used directly to control the display device.
The subfield encoding step is generally done after a degamma function as shown in FIG. 1. The degamma function is first applied to the input video levels. These levels are then coded by the sub-field encoding step into subfield code words. The subfield encoding step is eventually preceded by a dithering step. The subfield code words are then stored in a subfields memory.
In a standard approach, the encoding step is implemented by using a simple look-up table. A subfield code word is associated with each video level.
Some problems can not be solved at all or in a simple way when using this standard approach. This is the case of line load effect problem where the light emitted by a current pixel for a given video level can vary according to the load of the line of pixels to which the current pixel belongs. This problem can not be solved completely by using the standard approach. It is the same for the linearity problem when an average power level is controlled in the display device.
The line load effect is illustrated by FIGS. 2 and 3. The FIG. 2 shows a test picture (a white cross on a black background) to be displayed by a display device suffering from a problem of line load effect. The first and the last lines are black for one half of the pixels, and white for the other half. The middle lines are white. The FIG. 3 shows the picture as it is displayed by the display device. The line load effect is visible on the middle lines. This effect can be explained as follows: when a sub-field is used on a whole line its luminance is decreased by 20% compared to its luminance on a line where it is not used. The value of 20% is given as an example. The video level of the pixels of the middle lines is thus 255·(1−(1−½)×0.20)=229.5 while the white pixels of the other lines have a luminance of 255·(1−(1−1)×0.20)=255.
EP 1 768 088 discloses a recursive method to compute the sub-field code word from the bit associated with the most significant sub-field (sub-field having the highest weight) to the bit associated with the least significant sub-field (sub-field having the lowest weight). If the video level to be encoded is greater than or equal to a threshold associated with the sub-field, a state “ON” (or “1”) is allocated to the bit corresponding to this sub-field. The threshold associated with a given sub-field is the sum of the weights of the sub-field having a lower weight than the considered sub-field plus one.
This recursive method has a contour noise level similar to a standard coding without false contour optimization. This is due to the fact that each sub-field has a hard switch function i.e. a sub-field is not used at all if the video level to be encoded is lower than a threshold and is used completely for all the video levels equal to or greater than this threshold.